Adding a second attendee, which is the same contact (person) as the first attendee, but with a different email address, must always work
Some of my contacts have more than one email addresses and when I invite them for a meeting, I want to send an invitation to all their addresses.
I open an existing event without participants, select Options→Attendees, and type the first three letters of the contact. Then two choices are shown. I select the first address and this is the first attendee. Then I add a new attendee, type the first three letters of the same contact and two choices are shown.
Sometimes, things work as expected and the second attendee, which is the same contact as the first attendee, can be inserted in the dialog.
Sometimes, I cannot add a second attendee with a different address, if it is the same contact as the first contact and repeating the procedure does not change anything. I can add however a different contact as second attendee and then add the alternative email address of the first contact as third attendee. This way works.